Re: [PATCH 6/6] serial: 8250_dw: Ensure BUSY is deasserted

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jan 23, 2026 at 07:27:39PM +0200, Ilpo Järvinen wrote:
> DW UART cannot write to LCR, DLL, and DLH while BUSY is asserted.
> Existance of BUSY depends on uart_16550_compatible, if UART HW is
> configured with 16550 compatible those registers can always be written.
> 
> There currently is dw8250_force_idle() which attempts to archive
> non-BUSY state by disabling FIFO, however, the solution is unreliable
> when Rx keeps getting more and more characters.
> 
> Create a sequence of operations to enforce that ensures UART cannot
> keep BUSY asserted indefinitely. The new sequence relies on enabling
> loopback mode temporarily to prevent incoming Rx characters keeping
> UART BUSY.
> 
> Ensure no Tx in ongoing while the UART is switches into the loopback
> mode (requires exporting serial8250_fifo_wait_for_lsr_thre() and adding
> DMA Tx pause/resume functions).
> 
> According to tests performed by Adriana Nicolae <adriana@arista.com>,
> simply disabling FIFO or clearing FIFOs only once does not always
> ensure BUSY is deasserted but up to two tries may be needed. This could
> be related to ongoing Rx of a character (a guess, not known for sure).
> Therefore, retry FIFO clearing a few times (retry limit 4 is arbitrary
> number but using, e.g., p->fifosize seems overly large). Tests
> performed by others did not exhibit similar challenge but it does not
> seem harmful to leave the FIFO clearing loop in place for all DW UARTs
> with BUSY functionality.
> 
> Use the new dw8250_idle_enter/exit() to do divisor writes and LCR
> writes. In case of plain LCR writes, opportunistically try to update
> LCR first and only invoke dw8250_idle_enter() if the write did not
> succeed (it has been observed that in practice most LCR writes do
> succeed without complications).
> 
> This issue was first reported by qianfan Zhao who put lots of debugging
> effort into understanding the solution space.
> 
> Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
> Fixes: 7d4008ebb1c9 ("tty: add a DesignWare 8250 driver")
> Cc: <stable@vger.kernel.org>

Why is patch 6/6 only marked for stable?  If this is needed "now",
shouldn't this be a separate patch?  Do you need all of the first 5 for
this to work properly?

I can't take this series as-is because I don't know how to route it :(

thanks,

greg k-h